A typical Vertical Take Off and Landing (VTOL) vehicle may have one or more ducted fan units along or parallel to a longitudinal axis of the vehicle. Ducted fans have several advantages over free rotors (i.e., rotors not enclosed within ducts), the most prominent of which is the ducted fan's thrust augmentation that is attained when the incoming air accelerates over a suitably designed duct inlet lip, causing low pressure that acts upon the upper lip surface, creating up to 25-30% of additional thrust as compared to a free rotor.
Unfortunately, such augmentation comes at a price, mainly in the form of considerable rolling and pitching moments that are produced as a result of sideslip (i.e., sideward translational motion) or wind gusts.
VTOL vehicles that rely on ducted fans for lift and that need to operate in gusty wind conditions, must combat rolling moments that can interfere with the vehicle's operation and stability. One method that is advocated by Yoeli (U.S. Pat. No. 6,464,166 B1) is to use either a plurality of parallel, spaced, control vanes pivotally mounted to, and across, the inlet end of the duct, or a combination of two rows or groups of rotatable control vanes, one row at the duct inlet and one row at the duct exit (or outlet). These rows or groups of control vanes, each located at a distance from the center of gravity of the duct that is approximately half the depth of the duct (i.e., the vertical distance between the duct inlet and outlet), are, when rotated, able to produce rolling moments in a direction that is parallel to the vanes' axes of rotation, thereby opposing the adverse rolling moments caused by the lateral motion of the vehicle or alternatively, by a side-blowing wind when the vehicle is in hover. As this sideward motion increases, the vanes need to be rotated an increased amount until they reach the limit of the force that they can produce, with a consequent limit on the resistance to lateral wind or on the vehicle motion velocities. However, it has been discovered that adding vanes about the exit side of the duct may increase the total rolling moment acting on the duct.
Another way of enhancing the resistance to side winds is increasing the effectiveness of the control vanes as advocated by Yoeli (PCT/IB2009/055656) by increasing the distance between the upper (or inlet) control vanes and the vehicle's center of gravity and/or the lower (or outlet) row or group of control vanes, if installed, and designing the inlet side of the duct to affect the shape of the flow field in the vicinity of the duct inlet so that the flow into the duct can be made more uniform. With this added uniformity of flow, the control vanes, when mounted outside the duct, would be able to benefit from having a larger moment arm but would still be exposed to relatively high inflow velocities (a desired effect), in spite of being either completely or partially outside the boundaries of the duct.
Another way of coping with side winds as advocated by Yoeli (PCT/IL2007/001465) is designing selectively operated openings at the side walls of the duct preferably between the planes of the upper and lower vanes. The potential benefit of these side openings is the reduction of the sensitivity of the ducted fan to side airflow causing for example rolling moments. However, these methods have limits and a method is therefore desired for increasing the resistance of the duct to roll moments in order to enhance the ducted fan's ability to move sideways, perpendicular to the vanes' longitudinal axes or to resist side gusts and forces.